Pope Francis waves as he is driven through the crowd prior to his inaugural mass in St. Peter's Square at the Vatican today. Pope Francis thrilled tens of thousands of people on Tuesday, taking a long roundabout through St. Peter's Square and getting out of his jeep to bless a disabled man in a wheelchair in the crowd.

Dmitry Lovetsky/AP

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Vatican City — With traditional pomp and ceremony, the Catholic Church celebrated the official start of the papacy of Pope Francis on Tuesday, with a giant open-air mass in St. Peter’s Square.

Pope Francis, who was elected by a secret conclave of cardinals gathered in the Vatican's Sistine Chapel last week, was given a rapturous reception when he was driven around St. Peter’s Square in an open-topped popemobile at the start of the ceremony.

Under bright blue skies, Catholic faithful from around the world waved the flags of dozens of nations. The pope was accompanied by dozens of cardinals and archbishops wearing golden vestments that shone in the sunlight, as choirs sang Gregorian chants during the two-hour inauguration ceremony.

As tens of thousands of onlookers listened, many praised the dramatic change of style he has brought to the papacy, and waited for any hint of a major change for the Catholic Church in his first homily as pope. The new pope set out his vision for his papacy and the church as a whole by calling for greater compassion for the poor, the dispossessed, and the downtrodden.

“This was a general message, so he didn't get into specifics about how he might change the Church," says Robert Mickens, Vatican correspondent for The Tablet, a Catholic weekly.

"This is the big plan – mercy and compassion will be the keys to his pontificate. I think it's too early to say whether there will be a change of direction, says Mr. Mickens. “But his homily struck a more inclusive tone compared to Benedict XVI. He can use this as a launch pad to deal with other problems in the church. I suspect a lot of it was autobiographical – these are things he has learnt during his life about caring for the weak. He's a Jesuit, a bit of a lone ranger."

In a strongly worded homily delivered from a stage in front of St. Peter’s, he urged the world to shun "the omens of destruction and death."

He called on Christians “to protect the whole of creation, to protect each person, especially the poorest, to protect ourselves: This is a service that the Bishop of Rome is called to carry out, yet one to which all of us are called."

The vocation of protector

The duty of caring for the weak and the poor fell not just on Christians but on all of humanity, he said.

“The vocation of being a 'protector,' however, is not just something involving us Christians alone; It also has a prior dimension which is simply human, involving everyone. It means respecting each of God’s creatures and respecting the environment in which we live. It means protecting people, showing loving concern for each and every person, especially children, the elderly, those in need, who are often the last we think about."

He appealed to world leaders to make the protection of the environment a much stronger priority.

“I would like to ask all those who have positions of responsibility in economic, political, and social life, and all men and women of goodwill: Let us be 'protectors' of creation, protectors of God’s plan inscribed in nature, protectors of one another and of the environment. Let us not allow the omens of destruction and death to accompany the advance of this world," he said.

His message was very much in keeping with his decision, in the moments after his election in the Sistine Chapel last Wednesday, to take his name from St. Francis of Assisi, a symbol of humility and charity and a passionate lover of nature.

The mass was watched by some 100,000 people in St. Peter's Square and attended by dignitaries, heads of state, and royalty from around the world. Vice President Joe Biden represented the US, saying that President Obama would have attended had he not had a prior commitment to go to Israel.

Queen Elizabeth was represented by a minor royal, the Duke of Gloucester. The most controversial head of state to attend was Robert Mugabe, the president of Zimbabwe, who has been accused of decades of human rights abuses.

The ceremony started at the tomb of St. Peter, beneath the main altar of St. Peter’s Basilica.

Pope Francis was presented with a gold-plated silver ring, known as the Fisherman’s Ring, which depicts St. Peter holding a pair of keys in one hand and a fishing net in the other – reflecting Christ’s description of him as a fisher of men.

He also received the pallium, a special band of white wool, which is placed around the shoulders of the pope as a symbol of his universal jurisdiction over the Church.

Informal style

In the few days since he was elected the first pope from outside Europe in nearly 1,300 years, Pope Francis has established an informal, spontaneous style that has delighted many Catholics.

He has pressed the flesh with jubilant crowds in Rome on several occasions, addresses the leaders of the Church as “my brother cardinals” rather than “my lord cardinals,” signaling a more informal approach, and ditched the papal limousine in favor of riding in an ordinary mini-bus to and from the Vatican on official visits.

On Tuesday, he ordered the Popemobile to stop in the middle of the huge crowd and climbed down to bless a severely disabled man, kissing him on the forehead. He kissed babies who were held up to him and shouted "Ciao" to well-wishers.

"We have seen that the pope loves a certain element of spontaneity," says Federico Lombardi, the Vatican spokesman, speaking to journalists on Monday. "It may well be that he adds unscripted phrases and observations as he speaks."

Standing in the spring sunshine and watching the pope pass in his white open-topped jeep, Roger Amadasun, a DJ from Nigeria who was visiting Rome, says: “He’s going to be a good pope and I believe he will bring about change. He’s come from a good place – for the first time we have a Pope from Latin America. I like his style; He is friendly and open.”

A few paces away from him Miranda Gasperi, from Rome, says: “I like him very much – he’s very humble. He’s attentive to the needs of the poor. He comes from a poor country, so he knows what he is talking about. I hope that he will bring great change.”

Challenging the Vatican

While Pope Francis, the former archbishop of Buenos Aires, has brought a dramatic change of style to the papacy, whether he brings a change of substance remains to be seen.

There are high hopes among many in the Catholic Church that he will take a more robust line against pedophile priests and the bishops who have protected them, and set about cleaning out the dysfunctional Curia, the bickering, intrigue-ridden governing body of the Church. The fact that Pope Francis is the first Jesuit pope in history has bolstered the hopes of reformers – the order is renowned for challenging the Vatican.

"It is surely important that the cardinals elected, as pope, a member of a religious order that is famous for challenging the Vatican,” says Jose Bento Da Silva, an expert on the Jesuits from Warwick University in the United Kingdom.

“No other religious order knows how to manage a global community like the Jesuits. That knowledge will be extremely relevant when it comes to the challenges Pope Francis faces," said Dr. Da Silva by e-mail. “Being of European descent, but born and raised in South America, Pope Francis is one of the few cardinals capable of bringing together different ways of being a Catholic.”

The election of a “Jesuit Pope,” who knows Europe and South America, Da Silva added, should lead to an interesting organizational period for the Church. "It is worth noting that, historically, whenever the Catholic Church has wanted to change, it first changed its leader, rather than its structure, or its processes.”